The cheapest all-round is probably going to be Pricelining two rental cars. Both of the cars don't have to be large, just one of them. The best bet for luggage space is a full-size; the most common car in the US rental fleet is the Chevy Impala, which has a positively cavernous trunk. The other car can easily be a midsize to hold 5 people for just the transfers to and from the airport.
Having a rental is useful for getting your maximum money's worth because of the ability to eat or shop offsite. If you are the cheap sort, eating breakfast in your room with items you picked up at a supermarket (including a styrofoam cooler for the gallons on milk!) should make your flinty heart glad.
BTW, I agree with the PP that many people mistakenly think that flying into SFB is a bargain, when usually it is negligibly cheaper. Not only is there large ground transfer cost to be added in, but the domestic airlines that fly there tend to have rather heftier-than-usual baggage charges as well. FWIW, 10 passengers will qualify you for a group rate on SWA, which sometimes isn't all that much cheaper, but tickets purchased under the Group rate are fully refundable, which is often a huge issue when buying air tickets for people other than immediate family.